Agency controlled units can open and close doors when orthogonally or diagonally adjacent to them; guards can only open and close doors when orthogonally adjacent to them. In addition, agents can install Shock Traps on all kinds of doors when orthogonally adjacent to them.
Usually, when agents or rescued Couriers or Prisoners open or close doors that guards can see, the guard will notice the door opening or closing on the side it was opened or closed on. This will happen even if the tile the door is adjacent to is in soft cover, as doors do not receive soft cover. Hacked drones and Archive Prism in disguise can open and close doors without causing guards to be interested.
Human guards, when not alerted, will usually leave doors in the condition they were in after they move through them. The only exception is when they open a door to perform a peek or when they are KOed, killed, or alerted before they have the chance to close the door. Drones and all alerted guards will always leave doors open when moving through them.
Guards, when alerted, can in some conditions destroy closed doors, rendering them permanently open and dealing 1 KO damage to any unit on the other side. Agents, Couriers or Prisoners KOed in this way will also start the next turn with 0 AP if they were not pinned before waking up. This can only happen when the guard's interest point is one tile beyond the door from the guard, and it usually happens because the guard noticed an agent close it. An Akuma Drone can also be hacked to “kick” down doors as well.
Lvl1 Security Doors are doors that cannot be opened until they are unlocked by a Lvl1 Security Passcard, Banks's Crypto Computer, or a Lock Decoder after 2 turns. Guards cannot unlock these doors, nor will they try to path through them until they are unlocked. Banks can unlock the doors when she is either orthogonally or diagonally adjacent to them. The Lock Decoder can be placed only by an agent who is standing orthogonally adjacent to the doors.
Vault Doors are doors that cannot be opened until they are unlocked by a Vault Access Card. They will only appear in Vault missions and some Cybernetics Lab missions. They will always be found in the Objective rooms of these missions, and need to be unlocked and opened to access additional loot.
Exit Doors cannot be opened until they are unlocked by a Exit Access Card. They appear in Endless modes from day 5 onwards, in all facilities. Unlocking the Exit Door is required to reach the exit room and complete the mission.
Two guard entrance doors block the way to guard entrance rooms. These doors cannot be opened by agency-controlled units, and automatically close once guards pass through them, even if a guard would not normally close a door in that situation. As such, it is not normally possible for an agent to enter guard entrances.
It is possible to set up a Shock Trap on these doors.
Navigating between rooms of a facility is a core part of Invisible Inc.'s gameplay. Doors are the most common bottlenecks in Invisible Inc., both for vision and for navigating the facility, so a lot of Invisible Inc.'s strategy comes down to using doors effectively - maximizing your vision and mobility, and minimizing that of the guards.
The best strategy for the player depends heavily on what game mode you are on. When playing a higher difficulty than you are used to, you may need to unlearn strategies that worked on lower difficulties.
On Experienced and higher difficulty, guards will have longer patrols that often go through a door, and they can patrol into the Starting room. This makes doors more dangerous to the player than on Beginner.
This means that you will always need to account for the possibility that a stationary guard is watching the door, even if you peek through it and listen for patrols. Because of this, on Expert Plus, you should avoid opening doors unless you are able to account for this possibility, even with a peek through the door. As a result, if you can account for this possibility, it can often be better to open the door (from a diagonally adjacent tile) without peeking, peek with the door open, then (if noticed) get to safety.